Soldering method

ABSTRACT

A soldering method, which secures reliable joints without using flux, is disclosed. Soldering is performed using an auxiliary connecting material capable of physically destroying and dispersing an oxide film, which is naturally grown on the surface of a main connecting material such as solder or brazing between two terminals, to realize a reliable solder wetting (a state of the main connecting material being fused and mixed). For an auxiliary connecting material, hydrocarbon such as n-tetradecane (C 14  H 30 ), for example, can be used. N-tetradecane boils between connection surfaces and cubically expands; energy generated therefrom physically disperses the oxide film and causes a fresh surface to be exposed; solders on both terminals are mixed; and an electrical and mechanical joint is securely achieved. The residue of the auxiliary connecting material is an insulating material, and therefore does not need cleaning. An excellent electrical joint which is moisture-resistant as well as highly reliable is attained.

CROSS REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of theprior Japanese Patent Applications No. 7-91862 filed on Mar. 24, 1995and No. 7-348710 filed on Dec. 18, 1995, the contents of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a soldering method. More specifically,the present invention relates to a method of electrically andmechanically connecting a substrate such as a rigid substrate and aflexible substrate, or a method of electrically and mechanicallymounting on a substrate a connected member such as an IC (integratedcircuit) chip, a discrete element and so on.

2. Related Arts

In recent years, there have been growing demands, in the installation ofelectronic products, for techniques of connecting and mountingcomponents which are to be put in place at microscopic intervals. In thepast, flux has been used in the soldering (which hereinafter implies anelectrical connection by means of soldering and brazing). The flux hasto be removed by cleaning after a connecting operation is finished sinceit is corrosive. In view of the environmental problems, however, therehas been increasing restrictions imposed on the use of cleaning agentswhich contain CFCs (chlorofluorocarbons). A cleaning operation issometimes difficult to be performed in the case of a soldering operationwhich has to be executed inside a package. Consequently, attempts havebeen made to implement soldering operations dispensing with cleaningoperations by using low active flux which does not require the cleaningoperation, or to perform a soldering operation in an inactive atmospherewithout using flux if it has to be executed in a location which does notallow a cleaning operation. In the cases like these where low activeflux is used or soldering operations are performed without using flux,it is difficult to secure a sufficient area for connection due to theeffect of an oxide film on the surface of solder, which has given riseto such problems as the soldering being insufficient in strength and theconnection being low in reliability, or voids occurring inside solder.

As an example of a soldering which will solve such problems as mentionedabove, there is disclosed in Japanese Patent Laid Open No. 6-226485 amethod in which a soldering is performed with a solder alloy coated withparaffin wax on its surface. Paying attention to the fact that the voidsare caused by an oxide film, the technique has employed a method ofcovering solder with paraffin wax so as to prevent an oxide film frombeing grown on the surface of the solder.

Although the above-mentioned technique shows a method for preventing anoxide film from being formed, and for keeping and using solder under acondition in which there is no penetration of water, but it does notdeal with soldering which has to be performed under a condition in whichthere is an oxide film formed on solder. In other words, solderingcannot be executed unless terminals have been solder-plated in advanceas in the case of connections between terminals, and in the event ofsolder surface being oxidized, solder coated with paraffine wax cannotbe used for connections between terminals.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method ofconnecting or mounting a base member and a connected member, which cansecure for them a reliable connection and an excellent insulation.

To solve the above-mentioned problems, a soldering method, whichcomprises a step of disposing a main connecting material composed of asoldering or brazing material and an auxiliary connecting material of aninsulating material between a base member and a connected member; and astep of heat treating the main connecting material to be fused while atthe same time causing the auxiliary connecting material to be cubicallyexpanded or vaporized, can be employed. According to this, the auxiliaryconnecting material is subjected to cubical expansion or vaporizationduring the heat treating, the cubical expansion or vaporization of theauxiliary connecting material causes an oxide film grown on the surfaceof the main connecting material to be destroyed, and thus the basemember and the connected member are electrically and mechanically joinedthrough the main connecting material.

Accordingly, unlike flux, the auxiliary connecting material physicallydisperses the oxide film on the surface of the main connecting materialwithout chemically dissolving it, which makes it possible for the basemember and the connected member to be electrically and mechanicallyjoined securely. Herein, in case the auxiliary connecting materialcomprises an insulating material having no functional group, theinsulation quality thereof is prevented from deteriorating after thesoldering operation, and thus it will not cause the insulation qualityaround an electrode joint to be reduced even if it is left in theperiphery of the joint.

The main connecting material may be coated on at least one of connectionsurfaces of either the base member or the connected member. In thiscase, as the auxiliary connecting material, a material capable ofvaporizing around at the melting point of the main connecting materialmay be selected to cause the auxiliary connecting material to bevaporized during the heat treating step. By means of this, the oxidefilm can be destroyed during the heat treating step to electrically andmechanically connect the base member and the connected member with highreliability, and the auxiliary connecting material can be vaporized awayafter soldering as well.

Also, as the auxiliary connecting material, a material mainly composedof such material whose boiling point is greater than the melting pointof the main connecting material and whose vapor pressure is 1/100 orover of the external pressure at a maximum temperature during the heattreating step may be selected to cause the auxiliary connecting materialto be cubically expanded during the heat treating step. By means ofthis, the auxiliary connecting material existing around the jointportion will get into a boiling condition, undergoing an abrupt cubicalexpansion. The energy generated by this cubical expansion willphysically disperse and destroy the oxide film which is staying asformed without being dissolved on the surface of the main connectingmaterial, thereby causing the non-oxidized section to be exposed toelectrically and mechanically connect the base member and the connectedmember with high reliability.

Furthermore, it may be possible for the main connecting material and theauxiliary connecting material to be kneaded and formed into a paste-likecondition to be applied to the joint portion between the base member andthe connected member. Besides enabling the base member and the connectedmember to be electrically and mechanically connected in a goodcondition, the formed paste also allows the base member and theconnected member to be fixed tentatively through the paste, which makesit easier for them to be mounted.

Furthermore, the use of hydrocarbon for the auxiliary connectingmaterial may fully vaporize the auxiliary connecting material at atemperature that the main connecting material is fused. More preferably,the hydrocarbon must be alkane, alkene or alkyne.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, features and characteristics of the presentinvention will be appreciated from a study of the following detaileddescription, the appended claims, and drawings, all of which form a partof this application. In the drawings:

FIGS. 1A, 1B and 1C are explanatory views showing the soldering methodaccording to a first embodiment in which an auxiliary connectingmaterial is applied to the surface of a plated solder;

FIG. 2 is a schematic sectional view showing the array of connectingterminals of the first embodiment;

FIGS. 3A and 3B are schematic sectional views showing the state ofresidue between connecting terminals;

FIGS. 4A and 4B are graphs showing the temperature dependency of vaporpressure of alkanes;

FIG. 5 is a plane view showing the outlet terminals of a flat paneldisplay (EL display) according to a second embodiment;

FIG. 6 is a schematic sectional view showing the configuration of athird embodiment;

FIG. 7 is a schematic sectional view showing the configuration of afourth embodiment;

FIGS. 8A and 8B are a schematic sectional view and a perspective view,respectively, showing the configuration of a fifth embodiment; and

FIGS. 9A and 9B are schematic plane views showing the configuration of asixth embodiment.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EXEMPLARY EMBODIMENTS

(First Embodiment)

A description shall be given below of this invention with reference toseveral embodiments thereof.

FIGS. 1A to 1C are typical explanatory drawings showing the cases insequential order in which a copper terminal (the first connectionsurface) 1 which is formed on a rigid substrate 6 made of glass or glassepoxy and a copper terminal (the second connection surface) 2 which isprovided on a flexible substrate (a connected member) 5 made of plasticfilm are connected by means of soldering. FIG. 2 is a schematicsectional view showing a disposition of the copper terminals 1 and 2.

In FIG. 1A, solder (main connecting material) 3 has been appliedbeforehand on the surfaces of each copper terminal 1 and 2, and thesurface of the solder 3 is covered with an oxide film 7 before beingconnected. FIG. 1A shows the copper terminals 1 and 2 in that conditionplaced one on top of the other so as to be connected. Before placing theterminals one upon the other, an auxiliary connecting material 4characteristic of this invention must be applied on the surface of thesolder 3 between the connection surfaces in the manner described later.The auxiliary connecting material 4 may also be applied to the sides ofthe copper terminals 1 and 2, not being limited to the condition asshown in FIG. 1A.

The auxiliary connecting material 4 used here is n-tetradecane (C₁₄H₃₀), a kind of hydrocarbon. The auxiliary connecting material 4 is aninsulating material in a liquid state at room temperature. If containedbetween the terminals as stated above, it will not cause trouble such ascorrosion, since it is a non-reactive material. It has amoisture-resistant quality preventing the external moisture. The solder3 is made of eutectic solder which is composed of 60 wt % of tin (Sn)and 40 wt % of lead (Pb). Described below is a method of the soldering.

(1) The surfaces of the copper terminal 1 on the rigid substrate 6 andthe copper terminal 2 on the flexible substrate 5 are coated with thesolder 3 of a thickness of approximately 1 to 20 μm by means of plating.Then, the surface of the solder 3 is applied with n-tetradecane (C₁₄H₃₀) by means of brushing.

(2) Then, as shown in FIG. 1A, the substrates 5 and 6 to be connectedare put in place, and the copper terminals 1 and 2 solder-plated andcoated with the auxiliary connecting material 4 are placed one upon theother facing each other. Heat is applied from above the flexiblesubstrate 5 by means of a heater bar (not shown). When the solderingtemperature has risen up to a degree higher than the melting point of290° C., the solder 3 is fused to connect the copper terminal 1 with thecopper terminal 2. On this occasion, the substrates 5 and 6 arepressurized at 2 to 40 kg/cm² so that the copper terminals 1 and 2 willnot be displaced.

(3) n-tetradecane (C₁₄ H₃₀) boils at a temperature of 252° C. at whichthe solder 3 is fusing. As a result, the n-tetradecane (C₁₄ H₃₀) whichis caught in molten solder 3 between the copper terminals 1 and 2abruptly develops into a cubical expansion. As shown in FIG. 1B, thisabrupt cubical expansion acts on the molten solder 3 to physicallydestroy the oxide film 7 on the solder 3 existing on the surfacethereof. By this process, the molten solders provided on the surfaces ofboth the copper terminals 1 and 2 are mixed to connect themelectrically, being soldered mechanically at a desired strength. Asshown in FIG. 1C, since the joint is effected by means of soldering withthe oxide film 7 (see FIG. 1A) existing in the space between the copperterminals 1 and 2 destroyed, it has a high reliability of connection.Obviously, the oxide film 7 at the sides of the copper terminals 1 and 2remains after they are connected together.

(4) Also, a greater part of the n-tetradecane (C₁₄ H₃₀) remains asresidue, after being soldered (not shown), on the surfaces of the copperterminals 1 and 2, and of the solder 3, as well as on the surfaces ofthe rigid substrate 6 and the flexible substrate 5, which exist betweena plurality of the copper terminals 1 and 2. Since the n-tetradecane(C₁₄ H₃₀) does not contain functional groups such as carboxyl group,hydroxyl group and amino group, it does not corrode the copper terminals1 and 2 as well as the solder 3 even under high temperature and highhumidity. Therefore, it is not necessary to execute a cleaning, aftersoldering, to remove this auxiliary connecting material 4. Furthermore,the residue is effective as a passivation film to prevent thepenetration of moisture from the surface of the solder 3 into the copperterminals 1 and 2, thus giving the joint a high reliability for a longperiod of time.

The sectional structures of the copper terminals 1 and 2 on theconnected rigid substrate 6 and the flexible substrate 5 are shown inFIG. 2. As can be seen from FIG. 2, the width of the copper terminal 2on the flexible substrate 5 is different from that of the copperterminal 1 on the rigid substrate 6. The width of the copper terminal 2is narrower than that of the copper terminal 1. It is so structured thatthe copper terminal 2 is disposed within the width of the copperterminal 1, that solder fillet 8 is formed by means of soldering whichextends from the copper terminal 2 having a smaller width toward copperterminal 1 having a larger width, and that the adjoining copper terminal1a and 1b on the rigid substrate 6 do not come in contact with eachother. FIGS. 3A and 3B show a further detailed state of the connectionperformed under this configuration using the auxiliary connectingmaterial 4 of this invention.

FIG. 3A is a typical sectional view showing a state in which theauxiliary connecting material 4 remains as residue in the periphery ofthe connected copper terminal 1 and 2. FIG. 3B shows a state in whichthe auxiliary connecting material 4 remains filled up in the entirespace between the connected copper terminal 1 and 2 and their adjoiningcopper terminal 1' and 2'. The auxiliary connecting material 4 remainingas residue is not only moisture-resistant. It also plays a role ofprotecting the periphery of the connection land of the copper terminal 1and 2 from pollutant or corrosive substances, thereby enhancingdurability.

The auxiliary connecting material 4 should meet the following twoconditions.

First, the auxiliary connecting material 4 has a vapor pressure tovaporize that is enough for it to develop into cubical expansion at atemperature at which the main connecting material 3 is fused. As shownin the temperature-vapor pressure characteristic curve L1 in FIG. 4B,the boiling point T3 of the material should be over the melting point T1of the main connecting material 3, and the vapor pressure of thematerial should be higher than the pressure P1, which is 1/100 of theexternal pressure (atmospheric pressure) P2, at a heated maximum workingtemperature T2 at which the main connecting material 3 such as solder isactually heated and fused. In other words, it is a desirable conditionthat the temperature-vapor pressure characteristic curve L1 passesthrough the hatched region (T1-T2-P2-P1) in FIG. 4B which is enclosed bythe melting point T1, maximum working temperature T2, external pressureP2, and pressure P1 which is equivalent to 1/100 of the externalpressure.

The heated maximum working temperature T3 of the main connectingmaterial 3 means the maximum temperature reached by it while itstemperature is going up or down with an elapse of time at a specifiedtemperature gradient when it is heated and fused by means of suchheating sources as a heater bar. Furthermore, since the main connectingmaterial 3 and the auxiliary connecting material 4 are disposed at aclose interval, their temperatures are almost identical. Usually, theheated maximum working temperature T2 of the main connecting material 3varies according to materials which it is composed of. It is determinedin terms of the mechanical strength and reliability of the mainconnecting material 3 after being connected. In the embodiment, it wasset at 290° C.

Since the auxiliary connecting material 4 having the temperature-vaporpressure characteristic curve L1 which passes through the hatched regionhas a vapor pressure higher than 1/100 of the external pressure(atmospheric pressure) P2 at the heated maximum working temperature T2at which it is actually heated and fused, it develops into an abruptcubical expansion or boiling condition, which results in destroying theoxide film on the surface of the main connecting material 3 such assolder as was described earlier, thereby enabling a soldered joint.

On the contrary, those auxiliary connecting materials 4 of which thetemperatures at the melting points T4 are lower than the melting pointT1 of the main connecting material 3, in other words, those auxiliaryconnecting materials 4 of which vapor pressure characteristic curveshave the temperature-vapor pressure characteristic curve L2 higher thanthe external pressure at the melting point T1 of the main connectingmaterial 3, will have vaporized for the most part before they reach theheated maximum working temperature T2 when they are actually heated andfused, thus being not able to act so as to destroy the oxide film on thesurface of the solder 3.

Also, those auxiliary connecting materials 4 of which the vaporpressures are lower than the pressure P1 which is equivalent to 1/100 ofthe external pressure P2, in other words, those which have thetemperature-vapor pressure characteristic curve L3, will have vaporpressures lower than 1/100 of the external pressure (atmosphericpressure) P2 even at the time they reach the heated maximum workingtemperature T2 when they are actually heated and fused, thus being notable to act so as to destroy the oxide film on the surface of the solder3.

Furthermore, for the auxiliary connecting material 4 being capable ofdeveloping forcefully into a cubical expansion at the heated maximumworking temperature T2, it is desirable that they have vapor pressureshigher than the pressure P3 which is equivalent to 1/10 of the externalpressure in the atmosphere a soldering is performed.

Second condition is that the auxiliary connecting material 4 has aninsulation quality free from functional groups.

If the auxiliary connecting material 4 contains functional groups suchas carboxyl group, hydroxyl group and amino group, those functionalgroups will corrode the surfaces of the main connecting material 4,solder 3, and of the copper terminals 1 and 2, thereby reducing thereliability of the joint. Particularly, the residue of the auxiliaryconnecting material 4 containing functional groups is detrimental tosecuring joints of high reliability for a long period of time.

Furthermore, since the main connecting materials 3 for use in solderingsuch as solder and brazer have various melting points T1 and heatedmaximum working temperatures T2, it is required to select auxiliaryconnecting material 4 that has the temperature-vapor pressurecharacteristic curves suitable for respective cases.

A description shall be given below about several embodiments whichsatisfy the above-mentioned conditions, referring to FIG. 4A.

As the main connecting material 3, the eutectic solder in thisembodiment has the melting point T1=183° C. and the heated maximumworking temperature T2=290° C. A typical material that satisfies theabove-mentioned conditions is hydrocarbon, and among alkanes (CH₃(CH₂)_(m) CH₃) that contain the n-tetradecane (C₁₄ H₃₀) there are suchas undecane (C₁₁ H₂₄), icosane (C₂₀ H₄₂) and triacontane (C₃₀ H₆₂). Thetemperature-vapor pressure characteristic curves of those three itemsare shown in FIG. 4A.

Those three representative items are not chemically reactive and freefrom functional groups such as carbonyl group, hydroxyl group and aminogroup. They are insulating materials. By the way, in thetemperature-vapor pressure characteristic curves, hydrocarbons whichhave the smaller molecular weights have the higher vapor pressures andmelting points. In the figure, the temperature at the boiling point isindicated at the point of intersection of the horizontal line P2 for thepressure in general of 760 mmHg, an atmospheric pressure when asoldering operation is performed, and the temperature-vapor pressurecharacteristic curve.

Additionally, there are alkanes (CH₃ (CH₂)_(m) CH₃) of m=9 to 28 thatcan be used as the hydrocarbons that meet the above-mentioned conditionsnot shown in FIG. 4A. Specifically, there are undecane (C₁₁ H₂₄),dodecane (C₁₂ H₂₆), tridecane (C₁₃ H₂₈), tetradecane (C₁₄ H₃₀),pentadecane (C₁₅ H₃₂), hexadecane (C₁₆ H₃₄), heptadecane (C₁₇ H₃₆),octadecane (C₁₈ H₃₈), nanodecane (C₁₉ H₄₀), icosane (C₂₀ H₄₂),henicosane (C₂₁ H₄₄), docosane (C₂₂ H₄₆), tricosane (C₂₃ H₄₈),tetracosane (C₂₄ H₅₀), pentacosane (C₂₅ H₅₂), hexacosane (C₂₆ H₅₄),heptacosane (C₂₇ H₅₆), octacosane (C₂₈ H₅₈), nanocosane (C₂₉ H₆₀) andtriacontane (C₃₀ H₆₂).

In the above-mentioned embodiment, the straight chain of n-tetradecane(C₁₄ H₃₀) was illustrated for example. However, in addition to thematerials mentioned above, substances having side chains ofcarbon-carbon linkage, alkenes having double bonds of carbon in theskeleton of hydrocarbon, alkynes having triple bonds, aromaticsubstances not having functional groups or cyclic hydrocarbons can alsobe used. Furthermore, in addition to hydrocarbons as organic substances,inorganic substances such as silicone oil may also be used.

Next, in order to examine the durability of the joints formed by usingthe method of this invention, a voltage of 65 V was applied between theconnecting terminals adjoining side by side and not being cleaned, whichwere formed using n-tetradecane (C₁₄ H₃₀) and icosane (C₂₀ H₄₂), andthey were left standing in an atmosphere of 65° C. and 95% RH to checkthe moisture-resistance thereof. For the sake of comparison, tests werealso conducted on the samples which were connected using a rosin-linelow active flux. As a result, it was found that the incidence of failurefor the samples compared with was 100%, while that for the samples ofthis invention was 0%, when tested for a period of 500 hours.

Comparisons of the pulling-off strength were conducted by means of atensile test on the joints formed by using eutectic solder with thematerials listed in the Table 1 applied as auxiliary connectingmaterials. The tensile test was carried out by pulling the flexiblesubstrate 5 in FIG. 1A upward with an end as a support. A tensilestrength when the joint was pulled off was determined, and convertedinto the strength per width of 1 mm of the connection surface of thejoint. For the sake of comparison, the joints which were identical inthe contour but formed by using a conventional rosin-line low activeflux were also included.

                  TABLE 1                                                         ______________________________________                                        Auxiliary connecting                                                                           Peeling-off strength                                         material         (kg/mm)                                                      ______________________________________                                        Nil              0.05                                                         Low active flux  0.20                                                         Hexane (C.sub.6 H.sub.14)                                                                      0.08                                                         Undecane (C.sub.11 H.sub.24)                                                                   0.20                                                         Tetradecane (C.sub.14 H.sub.30)                                                                0.22                                                         Icosane (C.sub.20 H.sub.42)                                                                    0.21                                                         Triacontane (C.sub.30 H.sub.62)                                                                0.19                                                         Pentatriacontane (C.sub.35 H.sub.72)                                                           0.09                                                         ______________________________________                                    

As a result, it was found that four items of alkanes apart from hexane(C₆ H₁₄) and pentatriacontane (C₃₅ H₇₂) have tensile strengthsequivalent to those of the conventional items. As shown in FIG. 4A,triacontane (C₃₀ H₆₂) has a vapor pressure of approximately 9.8 mmHg atthe maximum working temperature limit of 290° C. at the time of asoldering, which is slightly more than 7.6 mmHg, 1/100 of theatmospheric pressure (760 mmHg). From this result, it was found that theuse in a soldering of auxiliary connecting materials which have a vaporpressure equivalent to 1/100 of the external pressure contributes tomaking electrical and mechanical joints securely.

Hydrocarbons generally used as auxiliary connecting materials arecommercially available often in a form of mixture with the alkaneslisted in Table 1. It is time and cost consuming to purely refine anitem of alkane. Mixtures having the temperature-vapor pressurecharacteristic curves within the region (T1, T2, P2 and P1) indicated inthe above-mentioned FIG. 4B can be used as auxiliary connectingmaterials 4 if they are hydrocarbons having as a whole thetemperature-vapor pressure characteristic curves which fall within thecompatible range, even in the case, for instance, of them containinghexane or pentatriacontane which did not fall within the compatiblerange for the eutectic solder in FIG. 4A.

Furthermore, hexane (C₆ H₁₄) and pentatriacontane (C₃₅ H₇₂) areeffective as auxiliary connecting materials if they have a specifiedvapor pressure at a soldering temperature when a low temperature solderor a high temperature solder is used. Accordingly, although those twoitems of alkanes cannot be used for eutectic solder, hexane (C₆ H₁₄) maybe used for a low temperature solder and pentatriacontane (C₃₅ H₇₂) fora high temperature solder.

Furthermore, in the above-mentioned embodiment, the auxiliary connectingmaterial 4 may be applied to the region of the surface on the substratebetween the terminals. The auxiliary connecting material 4 may be in astate of hydrocarbon as it is, or in a state of hydrocarbon as dissolvedin solvent. The auxiliary connecting material 4 is applied by means ofsuch as a normal brush.

In the embodiment mentioned above, the copper terminals 1 and 2 weretaken up for connection surfaces with the solder 3 loaded on bothsurfaces. However, gold (Au) may be used as a material for connectingterminals. Effect of this invention remains the same if the solder 3 isloaded on one side only. Although in the case of the solder 3 beingloaded on one side only, the connection is to be made not with thesolder 3, but with a metal terminal, effect of this invention remainsthe same as far as generally used metal terminals are concerned.

(Second embodiment)

FIG. 5 is a schematic diagram of a case in which this invention isapplied to an outlet terminal 51 on a plane display panel of an ELdisplay or a liquid crystal display. A description shall be given on anEL display 50. The EL display 50 is a display illuminated when voltageis applied to a luminous layer formed on a glass substrate 52. Thevoltage is applied by electrodes disposed in a matrix configuration. TheEL display 50 has terminals, that is, the outlet terminals 51, forapplying voltage from outside to electrodes disposed in a matrixconfiguration, which are formed around the periphery of the glasssubstrate 52 in a numerous microscopic array. This outlet terminal 51 iselectrically connected to a control substrate 54, an external circuit,through a flexible substrate (FPC) 53.

As in the case of the first embodiment, soldering was performed to theoutlet terminal 51 by using n-tetradecane. The result of the solderingis the same as shown in the sectional structure in the above-mentionedFIGS. 3A and 3B, having the same effect and realizing joints of highreliability. Since n-tetradecane is an insulating material, insulationis retained between the outlet terminals. It is also humidity-resistantpreventing moisture from penetrating between the electrodes.Furthermore, existence as residue of high polymer molecule between theelectrodes is effective in having the substrates connected each otherand restrains the solder being separated from the electrodes.

Since the outlet terminals 51 form numerous electrodes at microscopicintervals, it is important to enhance the reliability of soldering.Thus, this invention is applied so that individual outlet terminals 51can be soldered securely.

When disposing the outlet terminals 51 on the EL display 50 at narrowintervals, it is important for them to be securely soldered so thatthere will not occur a short-circuit between two adjoining outletterminals during a soldering operation. The inventors of this inventionsucceeded in preventing the short-circuit in question by forming on theterminal 51 a solder fillet 8 shown in FIG. 2. Faulty soldering thatcould occur with the conventional flux was restrained from occurring bymeans of the soldering method of this invention. Furthermore, iteliminated the need of a cleaning process after soldering, therebycontributing to reducing the production time. Particularly, since theluminous layer of the EL display 50 is often deteriorated due tomoisture, a substantial benefit can be brought about by the eliminationof a cleaning process after soldering.

In view of the above, the connecting method of this invention issuitable for use in laying out wires at microscopic intervals. On thisaccount, this invention has a wide range of application, being able tobe used not only for a plane display panel but also for solderingsubstrates with one another which have terminals formed on them spacedat microscopic intervals. By using the connecting method of thisinvention, reliable joints can be achieved electrically and mechanicallybetween the control substrate 54 and the flexible substrate 53.

(Third Embodiment)

FIG. 6 is a typical sectional view showing the connection structurebetween a circuit substrate 13 and a QFP (Quad Flat Package) type flatpackage 12 having an integrated circuit inside the package made up ofresin material, with leads 17 coming from the four sides thereof.

On the circuit substrate 13 are provided connecting terminals 14 at apitch of 0.6 mm. The connecting terminals 14 are plated with eutecticsolder of a composition of Sn 60 wt %-Pb 40 wt %. They are furthercoated with flux as an auxiliary connecting material, which is composedof abietic acid 10 wt %, adipic acid 0.05 wt %, n-tetradecane 10 wt %and the rest of organic solvent (not shown in the figure). The lead 17of the flat package 12 is provided at a pitch of 0.6 mm. After mountingthe flat package 12 on the circuit substrate 13 so that the lead 17 islocated on the connecting terminal 14, the lead 17 is heated by means ofa heater bar up to 220° to 290° C., the solder 3 is re-flowed andpreferably pressurized up to 2 to 40 kg/cm², and the lead 17 iselectrically and mechanically connected to the connecting terminal 14.

By connecting the lead 17 to the connecting terminal 14 in this method,when the solder 3 is re-flowed, the n-tetradecane (C₁₄ H₃₀) in theabove-mentioned flux boils at the temperature that the solder 3 is fusedand abruptly develops into cubical expansion. Through the boiling of theflux with it interposed between the solder 3 and the lead 17, an oxidefilm formed on the surface of the solder 3 is physically destroyed, andthe adipic acid as an activator acts on the oxide films on the surfacesof the solder 3 and the lead 17 to chemically remove them, therebyenabling an electrical and mechanical joint of the lead 17 to theconnecting terminal 14 to be performed securely. Since the boiling ofn-tetradecane is utilized, the quantity of an activator such as adipicacid used for chemically removing an oxide film can be substantiallyreduced (0.05 wt % in this embodiment), allowing excellent insulatingquality to be achieved in the mounting of components.

By the way, since the remains as residue of activator can cause a faultyinsulation, it has had to be removed in the past by cleaning aftersoldering. However, this embodiment has an advantage that the cleaningoperation can be dispensed with because of a substantially reducedactivator.

Furthermore, the connection structure stated above is also excellent inmoisture resistance as shall be described later. A moisture resistancetest was conducted by applying a voltage of 14 V between the leads 17adjoining side by side on the flat package 12, which were mounted butnot cleaned, and they were left standing in an atmosphere of 65° C. and95% RH for a test period of 500 hours, revealing a result that theincidence of failure was 0%. Consequently, application of theabove-mentioned flux to connect the flat package 12 to the circuitsubstrate 13 contributes to making a connection structure which isexcellent in moisture resistance.

The composition of the flux is not necessarily limited to the onementioned above. For instance, abietic acid may be in a range of 0 to10wt %, adipic acid and n-tetradecane may be in a range of 0 to 0.05 wt% and 1 to 10 wt % respectively.

(Fourth Embodiment)

FIG. 7 is a typical sectional view showing the structure of the fourthembodiment. The feature of this embodiment is that soldering paste 18 isapplied on a connecting terminal 14 of a circuit substrate 13, and thesoldering paste 18 is re-flowed to connect the connecting terminal 14 toa lead 17, with the other aspects of the structure being the same as inthe case of the third embodiment.

The soldering paste 18 is a kneaded mixture of 90 wt % of solderingpowder of 20 μm in diameter, 9.95 wt % of n-tetradecane (C₁₄ H₃₀) and0.05 wt % of adipic acid. The soldering paste 18 is printed and appliedon the connecting terminal 14, and a flat package 12 is mounted on thecircuit substrate 13 so that the lead 17 is located on the connectingterminal 14. Then, the lead 17 is heated up to a specified temperatureby means of a heater bar to cause the soldering paste 18 to re-flow andget pressurized at a specified pressure, thereby connecting the lead 17to the connecting terminal 14 electrically and mechanically.

Like this, by connecting the flat package 12 using the soldering paste18, an equivalent effect as in the case of the third embodiment can beattained. The use of the soldering paste 18 has an advantage that itallows the lead 17 of the flat package 12 to be temporarily fixed in apaste state.

The composition of the soldering paste 18 is not necessarily limited tothe one mentioned above. For instance, the soldering powder may bewithin a range of 90 to 95 wt % at 10 to 150 μm in diameter. Also,n-tetradecane and adipic acid may be within a range of 4 to 9.95 wt %and 0 to 0.05 wt % respectively. This invention does not impose anyrestriction on the composition thereof.

(Fifth Embodiment)

FIGS. 8A and 8B are a typical sectional view and a perspective view,respectively, showing the structure of the fourth embodiment. Thefeature of this embodiment is that an IC bare chip 21 with its siliconsurface exposed and a circuit substrate 23 are electrically andmechanically connected.

On the upper surface 23a of the circuit substrate 23 are provided aplurality of connecting terminals 24 at specified intervals, and on thelower surface 21a of the IC bare chip 21 are provided a plurality ofconnecting terminals 25 at the same intervals as for the connectingterminals 24. The connecting terminals 24 and 25 are plated witheutectic solder 3 composed of 60 wt % of Sn and 40 wt % of Pb, and alsocoated with auxiliary connecting materials (not shown) such asn-tetradecane indicated in the first to third embodiments. Then, the ICbare chip 21 is mounted on the circuit substrate 23 so that theconnecting terminals 25 are located on the connecting terminals 24,heated to cause solder 3 to re-flow and get pressurized at a specifiedpressure, thereby connecting the connecting terminals 24 to theconnecting terminals 25. Like this, by connecting the IC bare chip 21,an equivalent effect as in the case of the first to third embodimentscan be attained.

The process has a merit that it can dispense with a cleaning operationafter soldering, and eliminate the problems in the past that ionimpurities are left on bare chips with exposed silicon (Ion impuritiesremaining on bare chips cause trouble such as short-circuits due to ionmigration).

This embodiment is so configured that the IC bare chip is connected tothe circuit substrate 23 by using the solder 3 and the auxiliaryconnecting material. The configuration may also be such that solderingpaste 18 is used to achieve a connection as shown in the fourthembodiment.

(Sixth embodiment)

FIGS. 9A and 9B are typical plane views showing the configuration of thesixth embodiment. The feature of this embodiment is that a tape carrierpackage 38 and circuit substrates 33˜35 are electrically andmechanically connected.

The tape carrier package 38 is configured with a insulating carrier tape30, an IC chip 31 which is disposed almost at the center thereof,connecting terminals 39 which are provided at specified intervals atboth sides of the carrier tape 30, and leads 32 which electricallyconnects the IC chip 31 to the connecting terminals 39. Furthermore, thecircuit substrates 33˜35 each have connecting terminals (not shown)provided at the same intervals as the connecting terminals 39. Theconnecting terminals 39 and the connecting terminals of the circuitsubstrates 33˜35 are plated with eutectic solder composed of 60 wt % ofSn and 40wt % of Pb (not shown), and also coated with an auxiliaryconnecting material (not shown) such as n-tetradecane as in the case ofthe fifth embodiment.

In this configuration, the tape carrier package 38 is mounted so thatthe connecting terminals 39 are located on the connecting terminals ofthe circuit substrates 33 and 34, heated up to a specified temperatureto cause the solder to re-flow, and get pressurized at a specifiedpressure, thereby connecting the connecting terminals 39 to theconnecting terminals of the circuit substrates 33 and 34. In this way,as shown in FIG. 9A, the connection is executed with both ends of thetape carrier package 38 supported by the circuit substrates 33 and 34.The connecting terminals 39 and the connecting terminals of the circuitsubstrates 33 and 34 are well connected, and an equivalent effect as inthe first to third embodiments can be attained. In addition, as shown inFIG. 9B, the configuration may be such that a connection is executedwith the tape carrier package 38 supported by the entire surface of thecircuit substrate 35.

In this embodiment, it is so configured that the tape carrier package 38and the circuit substrates 33˜35 are connected by using solder and anauxiliary connecting material. It may, however, be so configured thatthe tape carrier package 38 and the circuit substrates 33˜35 areconnected by using the soldering paste 18 of a composition indicated inthe fourth embodiment.

By the way, besides eutectic solder of tin and lead, solders of tin andzinc, tin and silver, tin and antimony, cadmium and zinc, lead andcopper, zinc and aluminum as well as cadmium and silver may also beused. For brazing materials, brazers of magnesium, aluminum, silver,phosphorus and copper, copper and brass, nickel, gold, palladium as wellas cobalt may be used.

While the present invention has been shown and described with referenceto the foregoing preferred embodiments, it will be apparent to thoseskilled in the art that changes in form and detail may be made thereinwithout departing from the scope of the invention as defined in theappended claims.

What is claimed is:
 1. A soldering method comprising the stepsof:disposing a main connecting material composed of a soldering orbrazing material and an auxiliary connecting material of an insulatingmaterial between a base member and a connected member; and heat treatingsaid main connecting material to be fused while at the same time causingsaid auxiliary connecting material to be cubically expanded or vaporizedso that an oxide film grown on a surface of said main connectingmaterial is destroyed, whereby said base member and said connectedmember are electrically and mechanically connected by means of said mainconnecting material.
 2. A soldering method according to claim 1, whereinsaid disposing step includes the steps of:coating at least one of afirst connection surface of a first connection part provided on saidbase member and a second connection surface of a second connection partprovided on said connected member with said main connecting material;selecting as said auxiliary connecting material a material capable ofvaporizing around at the melting point of said main connecting materialso as to cause said auxiliary connecting material to be vaporized todestroy said oxide film during said heat treating step; and disposingsaid auxiliary connecting material between said coated main connectingmaterial and the other surface opposite to said one coated with saidmain connecting material.
 3. A soldering method according to claim 1,wherein said disposing step includes the steps of:coating at least oneof a first connection surface of a first connection part provided onsaid base member and a second connection surface of a second connectionpart provided on said connected member with said main connectingmaterial; selecting as said auxiliary connecting material a materialmainly composed of such material whose boiling point is greater than themelting point of said main connecting material and whose vapor pressureis 1/100 or over of the external pressure at a maximum temperatureduring said heat treating step so as to cause said auxiliary connectingmaterial to be cubically expanded to destroy said oxide film during saidheat treating step; applying said auxiliary connecting material at leastto said one of said first and second connection surfaces; and placingsaid base member and said connected member so that said first connectionsurface and said second connection surface face each other.
 4. Asoldering method according to claim 1, wherein said disposing stepincludes the steps of:kneading said main connecting material and saidauxiliary connecting material to form a paste; applying said paste atleast to one of a first connection surface of a first connection partprovided on said base member and a second connection surface of a secondconnection part provided on said connected member; and placing said basemember and said connected member so that said first connection surfaceand said second connection surface face each other.
 5. A solderingmethod according to any one of claims 1 through 4, wherein saidauxiliary connecting material is an insulating material free fromfunctional groups.
 6. A soldering method according to any one of claims1 through 4, wherein said auxiliary connecting material compriseshydrocarbon.
 7. A soldering method according to claim 6, wherein saidhydrocarbon is one selected from the group consisting of alkane, alkeneand alkyne.
 8. A soldering method according to claim 5, wherein saidauxiliary connecting material comprises hydrocarbon.
 9. A solderingmethod according to claim 8, wherein said hydrocarbon is one selectedfrom the group consisting of alkane, alkene and alkyne.
 10. A solderingmethod according to any one of claims 2 through 4, wherein said firstconnection surface and said second connection surface have widthsdifferent from each other.